1. Field of the Invention
The present invention relates to a white balance correcting device, a white balance correcting method and a storage medium for correcting white balance, which are suited for use in a video camera.
2. Description of Related Art
White balance correcting devices for video cameras of these days are mainly arranged to use the outputs of image sensors without using external sensors. Some of the known white balance correcting devices are arranged to avoid the adverse influence of chromatic colors as follows. Color-difference signals R-Y and B-Y and a luminance signal Y obtained from a signal processing circuit are divided into a number of small blocks corresponding to a picture. The signals within each of these divided blocks are averaged. Then, color signal components close to white are extracted from the mean values thus obtained. The white balance is controlled by bringing the mean values of the extracted color signal components into zero (“0”).
FIG. 8 is a block diagram showing, by way of example, the arrangment of an image pickup apparatus having a conventional white balance correcting device. Referring to FIG. 8, an object image having passed through a lens 101 and an iris 102 is formed on an image sensor 103. The image sensor 103 outputs signals of primary colors R (red), G (green) and B (blue) obtained by photoelectric conversion. The R, G and B signals are sent respectively to A/D converters 104, 105 and 106 to be converted into digital signals. The R and B digital signals are respectively sent to white balance amplifiers 107 and 108 to have their gains controlled on the basis of control signals supplied from a microcomputer 115. The R and B signals processed by the white balance amplifiers 107 and 108 and the G signal from the A/D converter 105 are sent to a matrix circuit 109. The matrix circuit 109 is arranged to form a luminance signal Y and color-difference signals R-Y and B-Y from the R, G and B signals. The luminance signal Y and the color-difference signals R-Y and B-Y are sent respectively to D/A converters 110, 111 and 112 to be converted into analog signals. The analog signals thus obtained are sent to an encoder (not shown) which is arranged to convert these input signals into standard TV signals. The TV signals from the encoder are sent out from the encoder either to be displayed on a monitor or to be supplied to a magnetic recording apparatus. Some of such recording apparatuses are arranged to record these signals in the form of the digital signals without having them converted into the analog form.
Meanwhile, the signals Y, R-Y and B-Y from the matrix circuit 109 are also supplied to a picture dividing part 113. The picture dividing part 113 is arranged to divide one picture amount of each of the signals Y, R-Y and B-Y into 8 vertical sections and 8 horizontal sections to give a total of 64 blocks, as shown in FIG. 9. A mean value computing part 114 computes and obtains the mean value of each of the signals Y, R-Y and B-Y for every divided block. The 64 sets of the signals Y, R-Y and B-Y are sent from the mean value computing part 114 to the microcomputer 115. At the microcomputer 115, only the signals of such blocks that the values of the color-difference signal and luminance signal are within a certain range are extracted (i.e., the so-called white extracting process is performed), and only the signals thus extracted are integrated. This extracting range is as shown, by way of example, in FIG. 10.
FIG. 10 is a diagram showing the variation of vectors of the color-difference signals taking place as a result of changes in color temperature of an object of achromatic color with the signal R-Y on the ordinate axis and the signal B-Y on the abscissa axis. In a case where white balance has been attained when color temperature is 7000 K which corresponds to an outdoor condition, the color-difference signals are located at a point P1 in photo-taking indoors with an incandescent lamp (at about 3000 K). Conversely, when white balance has been attained indoors with the incandescent lamp, the color-difference signals are located at a point P2 in photo-taking outdoors (at about 7000 k). In other words, the changes of color-difference signals with the color temperature of an achromatic object take place within a hatched part shown in FIG. 10. Assuming that white balance is to be controlled within a practicable range of color temperature from 3000 K to 7000 K, the white balance control can be accomplished by using signals within an area represented by the hatched part shown in FIG. 10. Hereinafter, this area will be called a white extracting range. Further, since white balance obtained under the light of a fluorescent lamp which is tinged with green in spectrum is taken into consideration, a white extracting area employed generally somewhat spreads in the direction of the color G. Furthermore, restrictions are sometimes imposed on the luminance signal Y in addition to the restriction on the color-difference signals. For example, such a restriction is imposed on the luminance signal Y that the level of the luminance signal Y is required to be equal to or greater than 50 IRE which is 50% of standard luminance of the luminance signal Y.
The microcomputer 115 extracts only the signals of blocks in which the color-difference signals are within the above-stated white extracting area and the level of the luminance signal is at least 50 IRE, and then computes mean values of the thus-extracted color-difference signals. Then, the microcomputer 115 corrects the white balance by sending to the white balance amplifiers 107 and 108 such control signals that cause the mean values of the color-difference signals R-Y and B-Y to become “0”.
However, the conventional arrangement described above has presented the following problems.
While a shooting object having a white portion largely distributed presents no problem, it is apt to be difficult to accurately extract white from such an object that has a white part finely distributed on a picture. FIGS. 11 and 12 are diagrams showing the respective objects on the picture for the purpose of explaining this problem. FIG. 11 shows the state of a block-divided picture on which a large image of a person in white clothes appears with a background of chromatic color. In this case, a white part of the image largely exists within the divided blocks indicated by arrows in FIG. 11. Therefore, if the color-difference signals are averaged within each of the divided blocks, white can be accurately extracted.
FIG. 12, on the other hand, shows a small image of the person in white clothes on the picture. The white part of the image does not much exist within the divided blocks as indicated by arrows in FIG. 12. Therefore, the white color and the color of the background are commingled when the signals within each of the divided blocks are averaged. In that case, the white extraction cannot be accurately accomplished. Assuming that the background is in a green color of turfs, the white part of the object and the green of the background mix together to result in a light green color, which prevents accurate white extraction in the event of the small white part in the divided blocks as shown in FIG. 12, although the white extraction can be accurately accomplished in the case of large white parts as shown in FIG. 11.
FIG. 13 is a color-difference signal vector diagram showing how colors are commingling in the case of the object shown in FIG. 12. The white clothing point Pa and the green background point Pb are caused to commingle by the averaging of the inside of each divided block. At the time when the microcomputer 115 reads signals, the two different colors commingle into a light green color point Pc. Since this point Pc is located within the above-stated white extracting range, the microcomputer 115 attempts to correct and make this point Pc white. FIG. 14 is a color-difference signal vector diagram showing a state obtained by the white balance correction. After the white balance correction, the light green color point Pc has become a white-balance-corrected point Pc′. The position of the point Pc′ is corrected at the center of vectors. Then, the white clothing point Pa of the object is erroneously corrected to a point Pa′, and the green background point Pb is also erroneously corrected to a point Pb′, as shown in FIG. 14. As a result of this white balance correction, the white clothing of the object comes to be tinged with a purplish color while the green color of the background becomes a lighter green color. Such an erroneous color correction has presented a problem.
Another problem of the prior art lies in that, although the above-stated problem may be mitigated by arranging the picture to be divided into more finely divided areas, such an arrangement not only causes an increase in size of the circuit arrangement, but also makes a period of time required for computing processes longer.